Positioning can make a mobile device either gather the information about its position or provide accurate particular position. This technology brings forward many novel concepts and services, including location-sensitive billing, fleet tracking, package and personal tracking, mobile yellow pages, location-based messaging, route guidance, and providing traffic information. Positioning has already been applied in GSM and 3G networks, and it will be one of the most promising and important features of the next generation wireless systems.
WiMAX system is based on technologies of IEEE 802.16 family of standards, sponsored by an industry consortium called WiMAX Forum. The IEEE 802.16 family of standards specifies the air interface of fixed and mobile broadband wireless access (BWA) systems that support multimedia services. IEEE 802.16-2004 standard, which was also previously called 802.16d or 802.16-REVd, was published for fixed access in October 2004, cf. “IEEE Standard for Local and metropolitan area networks. Part 16: Air Interface for Fixed Broadband Wireless Access Systems”, IEEE 802.16e. And the standard has now been updated and extended to 802.16e standard for mobile access, mobile WiMAX. Therefore, mobile WiMAX is one popular candidate platform to provide these services introduced by positioning. With the introduction of new Position Computation Service (PCS) service providers in the mobile WiMAX market, increased competition is expected between service providers to attract customers. However, to the best of our knowledge, there is no feasible solution for mobile WiMAX positioning.
The most widely employed location technologies are radio location systems that attempt to locate a Mobile Station (MS) by measuring radio signals between the MS and a set of BSs. Radio location systems can be based on signal strength, angle of arrival (AoA), time of arrival (ToA), time difference of arrival (TDoA) or their combinations. And it can be classified as two approaches: network-based and terminal-based. For network-based approach. BSs measure the signals transmitted by MS and relay them to a central site for processing, while for terminal-based approach. MS uses signals transmitted by BSs to calculate its own position. Several known positioning methods will be described there below.
Known Approach 1: GPS
The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). GPS is based on ToA technology and needs line-of-sight (LOS) propagation of satellite signals. Unfortunately, it is not feasible to let every MS have GPS functionality. Therefore, the GPS based technology is out of scope of this proposal.
Known Approach 2: Signal Strength
Radio location using signal strength is a well known location method that uses a known mathematical model describing the path loss attenuation with distance. Since a measurement of signal strength provides distance estimation between MS and BS, the MS will lie on a circle centered at the BS. By using multiple BSs, the location of the MS can be determined.
For signal strength based location systems, the primary source of error is multipath fading and shadowing. Variations in the signal strength can be as great as 30-40 dB over distances in the order of a half wavelength. Signal strength can be indicated by Receive Signal Strength Indicator (RSSI). And MS can obtain an RSSI measurement from the downlink burst preambles. From a succession of RSSI measurements, the MS shall derive and update estimations of the mean and the standard deviation of the RSSI, and report them via REP-RSP messages. But since RSSI is a quantized value, many errors are still introduced. In summary, there is rarely application scenario of signal strength positioning approach in cell based mobile networks.
Known Approach 3: AoA
AoA technique estimates the MS location by first measuring the angles of arrival of a signal from a MS at several BSs through the use of antenna arrays. But in the absence of a LoS signal component, the antenna array will lock-on to a reflected signal that may not be coming from the direction of the MS. Even if a LoS component is present, multipath will still interfere with the angle measurement. It needs 8 antennas in the BS when array antennas being used, it increases the engineering difficulty. Because the array antenna is not suitable for the NLoS, mobile WiMAX usually does not employ the array antenna technique.
Known Approach 4: Time-Based Approach: ToA and TDoA
The time-based approach is based on estimating the ToAs or TDoA of a signal transmitted by MS and received at multiple BSs or the TDoAs or ToA of a signal received at multiple BSs by MS. In the ToA approach, the distance between MS and BS is measured by finding the one way propagation time between the MS and the BS. Geometrically, this provides a circle, centered at the BS, on which the MS must lie, i.e. the radius of the circuit is the distance between MS and BS. By using at least three BSs to resolve ambiguities, the MS's position is given by the intersection of the circles. FIG. 1 shows the ToA positioning method. As shown in FIG. 1, the distance between MS and BS1. BS2 and BS3 are R1, R2 and R3, respectively. According the definition above described, MS must lie on the circles centered at BS1, BS2 and BS3 respectively and the radius of which are R1, R2 and R3, so the point of the intersection of the three circles is the MS's position.
In the TDoA approach, differences in the times of arrival are used. Since the hyperbola is a curve of constant time difference of arrival of two BSs, the time differences define hyperbolas with focus at the BSs, on which the MS must lie. Hence, the location of the MS is at the intersection of the hyperbolas. FIG. 2 shows the TDoA positioning method. As shown in FIG. 2, the solid line is the hyperbola, the focus of which is at BS1 and BS3, and the constant time difference of which is the distance difference between the distance from MS to BS1 and the distance from MS to BS3, (R1-R3). The dotted line is the hyperbola, the focus of which is at BS1 and BS2, and the constant time difference of which is the distance difference between the distance from MS to BS1 and the distance from MS to BS2 (R1-R2). According to the definition above described, MS must lie at the point of intersection of the solid line hyperbola and the dotted line hyperbola.
It should be noted that LoS propagation conditions are still necessary to achieve high accuracy for the time-based method. However, it is not so sensitive for ToA and TDoA compared with other methodologies such as AoA and signal strength. What's more, there are many papers and proposals on how to tell NLOS from LOS such as Wylie's estimation 1 “The None-Line-of-Sight Problems in Mobile Location Estimation”, IEEE Trans. Aerosp. Electron. Syst. Vol 16, pages 748-753. Therefore, in the rest of the present invention, we will propose how to apply TDoA for mobile WiMAX system.